Turbines are generally shut down when conditions indicate that imminent damage may occur to the turbine. Such conditions include low bearing oil pressure, rotor overspeed, low control fluid pressure, surge conditions, and the like. The turbine may be shut down by closing a trip-and-throttle valve fluidly coupled to the process fluid inlet of the turbine. Since it is important for a trip-and-throttle valve to close as quickly as possible, the trip-and-throttle valve may be biased to close. During normal operations, a pressure is exerted on the trip-and-throttle valve by a hydraulic fluid to maintain the trip-and-throttle valve in an open position. During failure conditions, the trip-and-throttle valve is closed by diverting the hydraulic fluid to a drain rather than to the trip-and-throttle valve.
A voting valve system may be used to direct the hydraulic fluid to the trip-and-throttle valve during normal operations and divert the hydraulic fluid to the drain during failure conditions. Conventional voting valve systems require a trade-off between safety level and false trip rate. A one-out-of-two voting valve system has two control valves and causes the turbine to shut down when one of the two control valves trips. This configuration results in a relatively high safety level, as only one of the control valves must function properly and trip upon receiving a trip signal to shut down the turbine. However, this configuration also has a high false trip rate, as the turbine is shut down if only one of the two control valves experiences a false trip. A two-out-of-two voting hydraulic dump system causes the turbine to shut down when both of the control valves trip. This configuration results in a lower safety level as both of the valves must function properly and trip upon receiving a trip signal to shut down the turbine. However, this configuration also has a lower false trip rate as a false trip may only occur if both of the valves experience a false trip.
There is a need, therefore, for a voting valve system which combines a high safety level with a low false trip rate.